1. Field of the Invention
The present invention relates to a radiation imaging apparatus that performs preliminary imaging and main imaging in sequence, a control method of the radiation imaging apparatus, and a radiation imaging system.
2. Description Related to the Prior Art
In a medical field, a radiation imaging system, for example, an X-ray imaging system using X-rays is widely known. The X-ray imaging system is constituted of an X-ray generating apparatus for generating the X-rays, and an X-ray imaging apparatus for taking an X-ray image by reception of the X-rays. The X-ray generating apparatus includes an X-ray source for emitting the X-rays to a patient's body, a source controller for controlling the operation of the X-ray source, and an emission switch for issuing an emission start command of the X-rays. The X-ray imaging apparatus includes an X-ray image detecting device for detecting the X-ray image based on the X-rays passed through the patient's body, and a console for controlling the operation of the X-ray image detecting device and applying various image processes to the X-ray image.
In recent years, the X-ray image detecting device that uses a flat panel detector (FPD) instead of an X-ray film or an imaging plate (IP) becomes widespread. The FPD has a matrix of pixels each of which produces signal charge by an amount corresponding to the amount of the X-rays incident thereon and accumulates the signal charge therein. The FPD accumulates the signal charge on a pixel-by-pixel basis, and converts the accumulated signal charge into a voltage signal by its signal processing circuit. Thereby, the FPD electrically detects the X-ray image, and outputs the X-ray image as digital image data.
An electronic cassette (portable X-ray image detecting device) that has the FPD contained in a flat slim housing is in practical use. The electronic cassette is mounted not only on a specific imaging stand, but also on an existing imaging stand shareable between a film cassette and an IP cassette. Furthermore, the electronic cassette is sometimes used while being put on a bed or held by the patient himself/herself. The electronic cassette is sometimes brought out from a hospital for use in bedside radiography of a home-care patient or in an outside accident or natural disaster site in an emergency.
The X-ray image detecting device having an automatic exposure control (AEC) function is widely known. In such a device, the FPD is provided with a dose detection sensor for detecting an X-ray dose. If an integral value of the X-ray dose detected by the dose detection sensor reaches a predetermined threshold value, an emission stop signal is transmitted to the X-ray source to stop generation of the X-rays.
In the AEC, if the source controller delays performing X-ray stop operation, the image quality of the X-ray image is degraded and moreover the patient is exposed to an unnecessary dose. For example, in mammography, the time between the start of X-ray emission and the stop thereof is extremely short, of the order of 50 ms, and hence it is required to perform the X-ray stop operation as quick as possible immediately after application of a sufficient X-ray dose is ensured. However, in actual fact, the transmission delay and propagation delay between the source controller and the X-ray image detecting device tends to retard the X-ray stop operation.
Therefore, as a method for reliably performing the X-ray stop operation without using the AEC at the timing that the desirable X-ray image is obtainable, a technique is proposed in which preliminary imaging is performed before main imaging, and an X-ray dose in the main imaging is controlled based on a result of the preliminary imaging. For example, in a radiation image capturing apparatus of U.S. Pat. No. 7,639,779, a radiation dose necessary for the preliminary imaging is calculated based on measured thickness of a breast and pressing pressure on the breast. Then, a radiation dose necessary for the main imaging is calculated based on an actual radiation dose detected during the preliminary imaging by a dose detection sensor. In each of the preliminary imaging and the main imaging, a tube current of an X-ray source is controlled in accordance with the calculated radiation dose. Japanese Patent Laid-Open Publication No. 2007-236804 discloses measuring pressing pressure on a breast being pressed by a board, determining a radiation dose for the preliminary imaging that is less than that for the main imaging in accordance with the pressing pressure, calculating an index value (the sum of pixel values of a breast area) by performing the preliminary imaging, and determining an exposure condition (current-time product) of the main imaging based on the difference between the index value and a predetermined reference value and a table representing the relation between the difference and a time-current product.
However, the U.S. Pat. No. 7,639,779 takes time and effort to measure the thickness of the breast and the pressing pressure on the breast, for the purpose of determining the radiation dose for the preliminary imaging. Likewise, the Japanese Patent Laid-Open Publication No. 2007-236804 also needs time and effort to measure the thickness of the breast and calculate the radiation dose for the preliminary imaging based on the measured thickness and to calculate the index value by analysis of an image obtained in the preliminary imaging.